Fuel injectors useful for applications such as gas turbine combustion engines direct pressurized fuel from a manifold to one or more combustion chambers. Fuel injectors also function to prepare the fuel for mixing with air prior to combustion. Each injector typically includes an inlet fitting connected to the manifold, a tubular stem or extension connected at one end to the fitting, and one or more spray nozzles connected to the other end of the stem for directing the fuel into the combustion chamber. A fuel passage (e.g., a tube, conduit or bore) extends through the stem to supply the fuel from the inlet fitting to the nozzle. Appropriate valves and/or flow dividers can be provided to direct and control the flow of fuel through the nozzle.
The fuel injectors are often placed in an evenly-spaced annular arrangement to dispense (spray) fuel in a uniform manner into the combustor chamber. Additional concentric and/or series combustion chambers each require their own arrangements of nozzles that can be supported separately or on common stems. The fuel provided by the injectors is mixed with air and ignited, so that the expanding gases of combustion can, for example, move rapidly across and rotate turbine blades in a gas turbine engine to power an aircraft, or in other appropriate manners in other combustion applications.
Because of limited fuel pressure availability and a wide range of required fuel flow, many fuel injectors include primary and secondary fuel flows to primary and secondary discharge orifices in each nozzle, with both the primary and secondary fuel flows being used during higher power operation (e.g., during cruise and take-off), and only the primary fuel flow being used during lower power operation (e.g., during idle descent). The secondary flow is significantly reduced or substantially or completely suspended during the lower power operation. Such injectors can be more efficient and cleaner-burning than single flow fuel injectors, as the fuel flow can be more accurately controlled and the fuel spray more accurately directed for the particular combustor requirement.
The primary and secondary flows in these types of injectors can be directed through primary and secondary passages supported in adjacent relation to one another, as in U.S. Pat. No. 4,735,044; or located in concentric relation to one another, as in U.S. Pat. No. 5,413,178. The concentric arrangement generally results in a more compact nozzle. In U.S. Pat. No. 5,413,178, for example, which is owned by the assignee of the present application, the fuel travels in concentric conduits where the primary flow from a pilot nozzle is routed through a cooling circuit down and back in surrounding relation to the primary and secondary flows for a main nozzle; and then down in surrounding relation to the secondary flow for the pilot nozzle. The secondary discharge orifice for both the pilot and main nozzles has an annular configuration and outwardly surrounds respective central primary discharge orifices. The primary flow from the pilot nozzle is constantly flowing during engine operation, and provides a sheath of cooling fluid for the fuel passages in the nozzle stem. Radial fins are provided between the inner fuel conduits for the secondary flows and the outer conduit defining the cooling circuit for the respective flow, for structural support and thermal management.
A fuel injector typically includes one or more heat shields surrounding the portion of the stem and nozzle exposed to the heat of the combustion chamber. The heat shield(s) are necessary because of the high temperature within the combustion chamber during operation and after shut-down, and to prevent the fuel from breaking down into solid deposits (i.e., “coking”) which occurs when the wetted walls in a fuel passage exceed a maximum temperature (approximately 400° F. (200° C.) for typical jet fuel). The deposits can build up and restrict fuel flow through the fuel passage and the fuel nozzle, rendering the injector inefficient or unusable.
As is known, the heat shield assemblies can take up valuable space in and around the combustion chamber, block air flow to the combustor, and add weight to the engine. They can also be labor-intensive, time-consuming and expensive to assemble and repair. Large, bulky heatshields can be undesirable with current industry demands requiring reduced cost, smaller injector size (“envelope”) and reduced weight for more efficient operation. In addition, certain applications require the secondary fuel conduit to surround the primary fuel conduit. In these applications, the cooling effect of a constant and significant fuel flow around the nozzle stem is absent (particularly during idle descent), and even more extensive heatshielding is necessary to avoid coking. U.S. Pat. No. 4,735,044, for example, suggests i) spacer wires between the outer, secondary conduit, and the outer heat shield; ii) “floating” the fuel tubes with respect to one another and to the heat shield so that there is no contact along their length; and iii) directing excess fluid between the secondary fuel conduit and the outer heat shield. As can be appreciated, such efforts add still further cost and complexity to the fuel injector.
Thus, it is a continuing challenge to develop fuel injectors to properly deliver fuel to a combustion chamber for operation of the gas turbine engine, where the injectors do not require bulky heatshielding and prevent, or at least reduce, coking in the injector during all aspects of engine operation. In applications where a secondary (non-continuous or reduced) fuel flow surrounds a primary (continuous) fuel flow, it is believed that the need for such injectors is even more pronounced.